Water/oil resistant paper (hereinafter referred to as fluorinated water/oil resistant paper) obtained by treating paper with a water/oil resistant composition containing a fluorinated copolymer having perfluoroalkyl groups (hereinafter referred to as RF groups), has a barrier property against a liquid such as water or oil and thus is used as e.g. wrapping paper for food products or a packaging container for food products. Further, fluorinated water/oil resistant paper has good water resistance and oil resistance while maintaining air permeability, as different from water/oil resistant paper other than the fluorinated type, and thus, it is used also as wrapping paper for fried products which disfavor moisture remaining inside, as wrapping paper for a freshness-keeping agent which requires air permeability for its function, or as wrapping paper for a deoxidant.
The following two types of methods are available as a method for treating paper with a water/oil resistant composition.
(1) A method so-called “internal sizing” wherein a water/oil resistant composition is added to a pulp slurry before paper-making.
(2) A method so-called “external sizing” wherein a water/oil resistant composition is applied to paper in size press coating after paper-making, wet pressing and preliminary drying, or a water/oil resistant composition is applied to paper after preparation of base paper by means of various coaters or various printing machines.
As a water/oil resistant composition containing a fluorinated copolymer, the following one is, for example, known.
A water/oil resistant composition having a fluorinated copolymer dispersed or dissolved in an aqueous medium, wherein the fluorinated copolymer is one obtained by converting amino groups with an acid to form a salt in a fluorinated copolymer obtained by copolymerizing a (meth)acrylate having a polyfluoroalkyl group (hereinafter referred to as an Rf group), a dialkylaminoalkyl (meth)acrylate and a vinyl monomer such as vinyl acetate (Patent Document 1).
In such a water/oil resistant composition, as the acid to be used for conversion of amino groups to form a salt, acetic acid is employed from the viewpoint of the safety and price.
However, the water/oil resistant composition obtained by using acetic acid for conversion of amino groups to form a salt, has the following problems.
(i) The fluorinated copolymer having amino groups converted to form a salt is cationic, and therefore, if an anionic adjuvant is used at the time of treating paper, not only no adequate water/oil resistance can be obtainable, but also agglomerates are likely to be formed thus leading to a trouble in the operation.
(ii) In the external sizing, there may be a case where water having a high pH (e.g. water having a pH of at least 8) is used as dilution water, or in the internal sizing, there may be case where a pulp having a high pH is used. Also in such a case, not only no adequate water/oil resistance can be obtainable, but also agglomerates are likely to be formed, thus leading to a trouble in the operation.
(iii) There is an odor specific to acetic acid.
In recent years, there has been a concern about an environmental load due to fluorinated compounds having RF groups with at least 8 carbon atoms such as perfluorooctanoic acid and its precursor, analogues, etc., and an attempt is being made by e.g. US Environmental Protection Agency, to reduce their accumulation in the environment. Therefore, a substitute water/oil resistant composition is desired which contains a fluorinated copolymer having RF groups with at most 6 carbon atoms or perfluoropolyether (Patent Document 2).
However, a substitute water/oil resistant composition obtained by converting amino groups with acetic acid to form a salt in a fluorinated copolymer having RF groups with at most 6 carbon atoms or perfluoropolyether, is inadequate in the water/oil resistance, as compared with a conventional water/oil resistant composition obtained by converting amino groups with acetic acid to form a salt in a fluorinated copolymer having RF groups with at least 8 carbon atoms.